DE69118185T2 - Procedure for creating the location and arrangement of an object under investigation - Google Patents

Description

The invention relates to a method and a device for determining the position and/or the configuration of an object under observation.

Conventional methods for determining the configuration of an object use a coaxial method in which a laser light beam is incident on the object and reflected light is captured by a camera and a position and configuration of the object are determined.

Figure 10 illustrates the principle of the conventional coaxial method. As shown in the figure, a laser light beam 5 falls on an object 2 and is reflected to the light receiving section of a detector 3 (camera). From known positions and angles of the laser source 1 and the camera 3, a position and a configuration of the object are calculated.

Although the method is suitable for very accurate measurement, a detector 3 must be adjusted so that its optical axis is coaxial or coincident with the direction of the propagating source, which is often difficult and is only applicable to objects which have a special shape and are made of a special material.

If the object does not reflect any incident light at all, it is difficult to measure the surface of the object because there is no reflected light. And in the case of a complex object surface, it is often difficult to measure the surface of the object for the reason that the Cameras cannot be adjusted in the direction of the reflected light.

Another difficulty with optical methods is mainly due to the fact that they rely on information concerning the state of the object surface and do not take into account all the information concerning the light beam itself.

US-A-4 289 397 describes an apparatus for processing a laser cellometer signal. In particular, a method and apparatus is described for determining the position of a plurality of points on a target, e.g. a cloud, using a laser source for emitting a laser beam and a camera which is not coaxially arranged with respect to the laser beam path, wherein a light scattering medium scatters the laser beam.

EP-A-0 071 426 discloses a method and a system for determining a shape in a plane to be determined, in an atmosphere of scattering materials. Specifically, this document describes a method and an apparatus for determining three-dimensional position coordinates of a shape in a plane, using a laser source for emitting a laser beam, a light scattering medium for scattering the laser beam and a camera for capturing the scattered laser beam light. The camera is not necessarily coaxially oriented with respect to the laser beam path, the position of a plurality of points in the plane is determined, and the light scattering medium is, for example, haze.

It is therefore an object of the present invention to provide a method for determining the configuration as well as the To create surface condition of an object by measuring the scattered location (locus) of the laser beam.

This object is achieved according to claim 1.

Preferred embodiments are set out in the dependent claims.

Figure 1 is a schematic diagram of the arrangement of a laser mounted on a mandrel of a machining tool and an object located on an NC table of the tool for observation.

Figure 2 illustrates a mode of the laser beam process and a structure suitable therefor according to the invention.

Figure 3 illustrates another mode of the method of the invention in which the unknown direction of the laser beam emitted by a laser is determined by determining two points on the beam by a camera, from which points the position of the object is determined.

Figure 4 illustrates another mode of the method of the invention in which the position of the laser is unknown and a point on the beam is determined to determine the configuration of the object.

Figure 5 illustrates another mode of the method of the invention in which the configuration of an object is determined from the normal vector at that position by observing the ray incident on and reflected from the position.

Figure 6 illustrates the use of this invention to determine the configuration of a workpiece immersed in a liquid during electro-erosive machining.

Figure 7 is an illustration to explain the effect of the refractive index of a liquid which plays a role in the inventive method.

Figure 8 illustrates how light scattering can be used in the method according to the invention.

Figure 9 illustrates a method according to the invention in which a layer of oil is applied to an object having a mirror-like surface and a laser beam is directed into the film to observe the light emerging from the film to determine the configuration of the object.

Figure 10 illustrates a conventional method using a laser beam.

Figure 1 illustrates a method for determining the configuration of a workpiece 2 located on a numerically controlled (NC) work table (not shown) of a machine tool by means of a laser 1 mounted on a mandrel of the machine tool. The laser beam may be projected onto the object through an optical fiber. A reflected laser beam 5 is detected by an observation camera 3, which may be an image recognition device including a CCD camera.

The laser light beam is made visible as it passes through a medium 4 which contains microscopic particles (e.g. smoke, saturated water vapor) and scatters light. Other light-scattering media such as colloidal liquids, polymer liquids, colored liquids, powder-containing liquids, powder-containing air, and colored air can also be used. Instead of the particle-containing media, a liquid with a maximum scattering intensity for the given wavelength of the laser light can be used, such as acetone. If no such light-scattering medium is available, an atomizer 6 can be used to create microscopic particles in the path of the laser light beam. It is obvious that such an atomizer 6 is not necessary if the measurement is carried out in a liquid or gas which can scatter the laser light beam.

No external microscopic particles are necessary if such particles actually exist in the measurement environment. The laser 1, camera 3, and atomizer 6 can be designed compactly so as to be integrally mounted on the mandrel. In a light-scattering medium as described above, the laser beam 5 becomes bright due to Rayleigh scattering and can be observed from any direction. By determining the path or locus of the laser beam, a point B on the surface of the workpiece at which the laser light is reflected is determined. By scanning the surface of the workpiece and determining other points B on the surface in a similar manner, the configuration of the workpiece can be obtained.

Figure 2 shows the concept of the invention in which a laser 1 emits a laser beam 5 which impinges on an object 2 and is reflected to a camera 3. An atomizer is not shown in this figure. The medium through which the laser beam travels is densely populated with particles which can cause Rayleigh scattering. Thus, the laser beam 5 emitted by the laser source 1 can be observed impinging on a point B of the object 2 and recognized as a trace between A and B. The position vector Rb of the point B as observed from any point of the reference point 0 is described by equation (1).

Rb = RL + d nL = Rc + c nb (1)

where RL and Rc are the position vectors of the laser source 1 and the camera 3 observed from point 0. nL is the unit vector pointing in the direction of the laser beam emitted by the source. nb is the unit vector pointing from the camera to point B. From equation (1) we obtain the equation (2) below.

d nL - c nb = Rc - RL (2)

The vectors nL, nb, Rc and RL can be obtained from measuring the positions of the origin 0, the laser source 1, the object 2, and the camera 3. By simultaneously solving the equations (1) and (2), the values of d and c can be obtained. By substituting the values of d and c, the vector Rb is determined.

It should be noted that equations (1) and (2) are actually three equations for two unknowns. Therefore, one can arbitrarily choose two equations or calculate the minimum error solution using a generalized inverse matrix.

Figure 3 illustrates a method for determining the unit vector nL, which is useful in the case where the direction of the laser source 1 is not known. In this method, arbitrary points A1 and B on the laser beam 5 are determined by means of the camera 3. The atomizer is not shown.

The laser beam emitted by the laser source 1 provides the trace of the beam between the points A₁ and B, so that the distances d₁ and d₂ from the camera 3 (at position C) to the points A₁ and B can be determined. The following vector relations (3) and (4) are valid with respect to

Point A₁: a + RL = Rc + d1 nb1 (3)

and point B: αa + RL = Rc + d&sub2;nb2; (4)

where nb1, nb2 are unit vectors pointing from point C of camera 3 to points A and B, a is the vector pointing from point A, the position of the laser, to point A₁, αa is the position vector of point B, seen from point A₁, d₁ is the distance between points C and A₁, and d₂ is the distance between points C and B. One can consider point A as the position of the laser and point B as the point of the object. Equations (3) and (4) lead to equation (5).

a (Rc - RL) + αd₁ nb1 - d2 nb2 = Rc - RL (5)

This is a simultaneous equation for αd1, and d2, which also gives d1. By substituting the value of d from equation (6) below, a is determined. The unit vector nL, the unit vector of the laser beam emitted by the laser, is given by equation (7).

a = Rc - RL + d1 nb1 (6)

nL = a / a (7)

On the basis of this unit vector nL, the vector Rb of the point B viewed from the origin 0 can be determined in the same way as discussed in connection with Figure 2.

Figure 4 shows a method that can be used to determine the position of a point B on the laser beam 5 without knowledge of the position of the laser source 1, on the basis of the known direction of the emitted laser beam. A first camera 3a and a second camera 3b are used to observe the point B. The relationship between the position C₁ of the first camera 3a and the position C₂ of the second camera 3b with respect to the origin 0 is given by

d1; nb1 + RC1 = d2 nb2 + RC2 (8)

where RC1 and RC2 are the position vectors of the cameras 3a and 3b, respectively, nb1 and nb2 are the unit vectors pointing from the respective cameras to point B, d1 is the distance between the points B and C1, and d2 is the distance between the points B and C2.

Equations (8) and (9) lead to

d1; nb1 - d2; nb2 = RC2 - RC1 (9)

This simultaneous equation gives d₁ and d₂, which in turn gives the position vector Rb of the point B considered from the origin 0, according to an equation:

Rb = d1 nb1 + RC1 (10)

Point B can be chosen on the surface of the object of interest to determine the configuration of the object.

Figure 5 shows the geometric relationship among the vectors used in a method for determining a normal unit vector nn on the surface of an object by determining a unit vector nr representing the laser beam reflected from point B on the object and a unit vector nL representing the beam incident on point B. The vector nn is expressed in terms of nr and nL as follows.

nn = nr - nL / nr - nL (11)

In conventional methods, rapid determination of such normal vectors mentioned above is difficult because even when many representative points on the surface of the object are obtained, calculations are required to determine them. In contrast, the invention can easily provide the normal vectors nn.

In cases where the surface under observation has a rough surface, the laser beam reflected therefrom has a Gaussian distribution in intensity as shown in Figure 5. Accordingly, the maximum intensity direction P of the reflected beam is obtained by setting a threshold value when processing the image data. The measurement accuracy can therefore be improved by obtaining the continuous point of maximum light quantity.

As described above, the normally reflected beam has a spatial intensity distribution around the vector nr as shown in Figure 5. Thus, it is possible to determine the roughness of the surface by measuring the spatial intensity distribution. This measurement also allows the detection of scars and sharp edges on the surface along with the detection of the roughness of the surface.

Figure 6 illustrates the use of a laser beam 1 to determine the configuration of a workpiece 2 in an electro-erosion machining vessel 8. A typical electro-erosion machining is carried out in an oil 9 held by the vessel 8. Since such a workpiece 2 is immersed in oil 9 or water, which scatter the laser beam, the configuration and roughness of the workpiece can be obtained in the manner described above.

The use of mirrors and an optical fiber will facilitate the measurement and can allow a determination of the configuration and roughness of the electrode, as the laser beam reaches every point of the electrode. It can also allow a determination of the configuration of the electrode in real time and can change the processing condition based on the measurement result. In this case, the measurement can be carried out at a practically constant temperature, since the heat capacity of the liquid (oil or water) is very high.

It should be understood, however, that the refractive index of the liquid must be taken into account in the above-mentioned measurements. The true distance D between the free surface of the liquid and the surface of the workpiece is given by D = n d, where n is the refractive index of the liquid and d is the apparent distance, as shown in Figure 7.

Figure 8 shows a method for determining the configuration and position of an object 2 by using the scattering of a laser beam 5 emitted by a laser 1. The object is in an atmosphere rich in dispersed light scattering particles (such as smoke and saturated water vapor). The scattering of the laser beam incident on the surface of the object 2 is observed by the camera, which is not coaxial with the laser beam. The point B where the beam 5 is reflected can be observed as a very bright spot, so that by determining the brightest point in the spot the position of the surface is determined.

Figure 9 illustrates the use of an oil film applied to the surface of an object. The oil is selected from materials that can scatter laser light. This method allows the configuration of an object that has a mirror-like surface to be determined. When a laser beam is introduced into one end of the oil film, the laser beam will propagate through the film, is reflected multiple times in it. By observing the intensity of the laser light emerging from the film, the locus along the surface of the object is observed as a spatial area and the configuration of the object can be determined.

The intensity of the scattered light fluctuates because the heterogeneously distributed particles in the gas or liquid fluctuate. Accordingly, the accuracy of the measurement is improved by suppressing the measured value of the scattered light.

When the particles suspended in the gas or liquid move along the surface of the solid, by observing the movement of the suspended particles using a Doppler laser, the position on the solid is measured very precisely. This means that by forcing the flow of the fluid (gas or liquid) containing the suspended particles and measuring the direction of movement of the suspended particles, the configuration of the surface is determined.

According to the invention, the determination of the configuration of an object can be carried out by measuring the scattered locus of the laser beam or the scattered point where the laser beam strikes the object, it can even measure the surface of an object which does not reflect laser light at all. The method of this invention can be used regardless of the properties of the surface, so that the invention can be applied to both mirror-like surfaces and rough surfaces.

Claims (9)

1. Method for determining a position of a point B on an object (2), comprising the steps: Shining an incident laser beam emitted by a laser (1) onto the object, the object (2) directing the incident beam as a reflected beam (5), the incident and reflected beams defining a beam path; Providing a light-scattering medium (4) on the laser beam path; Detecting a locus of the laser beam by at least one detection device (3; 3a, 3b) which is located at a position outside the beam path; where the position of the point B, represented by a vector Rb, viewed from an arbitrary origin 0, is determined by detecting the scattered light of the laser beam, from an equation: Rb = RL + d nL = Rc + c nb where RL and Rc are the position vectors of the laser (1) and the detection device (3; 3a, 3b) respectively with respect to the arbitrary origin 0, nL is the unit vector pointing in the direction of the laser beam emitted by the laser (1), nb is the Unit vector pointing from the detection device (3; 3a, 3b) to the point B on the object (2), and c and d are the respective distances from the detection device and the laser to the point B on the object (2). 2. The method of claim 1, wherein the position vector Rb is determined by: Detecting a point A₁ and the point B on the scattered laser beam by the detection device (3; 3a, 3b), and Determination of the unit vector nL from the following equations: a + RL = Rc + d₁ nb1 αa + RL = Rc + d₂ nb2 a = Rc - RL + d1 nb1 nL = a / a where nb1 and nb2 are the unit vectors pointing in the direction from the detection device (3; 3a, 3b) to the respective points A₁ and B, a is the position vector of the point A₁ viewed from the laser (1), αa is the position vector of the point B viewed from the laser (1), and d₁ and d₂ are the respective distances from the detection device (3; 3a, 3b) to the points A₁ and B. 3. A method according to claim 1, wherein the position vector Rb is determined by detecting the point B on the scattered laser beam by a first and a second detecting device (3; 3a, 3b), from the following equations: d1; nb1 + Rc1 = d2 nb2 + Rc2 Rb = d1 nb1 + Rc1 where Rc1 and Rc2 are the respective position vectors of the first and second detection devices (3; 3a, 3b), nb1 and nb2 are the unit vectors pointing in the directions from the first and second detecting devices (3; 3a, 3b) to the point B on the object (2), and d1 and d2 are the respective distances from the first and second detecting devices (3; 3a, 3b) to the point B on the object (2). 4. Method according to claim 1, wherein the detection devices (3; 3a, 3b) are not arranged coaxially with respect to the laser beam path. 5. The method of claim 1, wherein the position of a plurality of points on the object (2) is determined. 6. The method of claim 5, further comprising moving the medium past the plurality of points. 7. The method of claim 6, further comprising the step of performing Doppler laser measurements of the moving medium. 8. The method of claim 1, further comprising the step of compensating for the refractive index of the light-scattering medium (4). 9. The method of claim 1, further comprising measuring the spatial intensity distribution of the reflected beam.